The present invention relates to a magnetic recording medium which is available for high density information recording and specifically to a magnetic recording medium suitable for high density magnetic recording.
Recently, in compact high capacity magnetic disk drives which are installed in both personal computers and home electric appliance there has been a strong demand to increase the capacity of the magnetic recording/reproducing apparatus and to improve the recording density. In order to respond to this demand, magnetic heads and magnetic recording media are being developed aggressively. However, it has become difficult to improve the recording density by using a longitudinal magnetic recording method which is the current commercial practice. Perpendicular magnetic recording has been studied as a method which will substitute longitudinal magnetic recording. Perpendicular magnetic recording has a stable high recording density state because adjacent magnetizations do not face each other. Moreover, the recording efficiency can be improved by combining a single pole type write head and a double-layered perpendicular recording medium having a soft magnetic underlayer, which allows for dealing with an increase in the coercivity of the recording layer.
In a perpendicular recording method, in order to partake in returning the recorded magnetic flux from a magnetic head, a soft magnetic underlayer basically has to be made of a relatively thick soft magnetic material having a high saturation flux density (Bs). Therefore, noise caused by the soft magnetic underlayer becomes a problem, and the improvements which have been proposed up to now deal essentially with the layer structure of the soft magnetic underlayer in order to decrease this noise and to improve the magnetic recording characteristics.
For instance, JP-A No. 103553/1994 proposes a method in which the soft magnetic underlayer consists of a triple-layered structure with a first soft magnetic layer, a non-magnetic layer, and a second soft magnetic layer, and the noise generated by the magnetic wall motion of the soft magnetic layer is inhibited by exchange coupling with the antiferromagnetic layer in which the direction of the magnetic spin is aligned. Moreover, JP-A No. 79058/2004 proposes a method to decrease the medium noise by oxidizing the surface of the soft magnetic underlayer, forming a soft magnetic underlayer consisting of a first soft magnetic layer, an intermediate layer, and a second soft magnetic layer thereon, and promoting the refinement of the magnetic particles of the perpendicular magnetic recording layer. Although different compositions containing Co are described for the first and second soft magnetic materials, they are for improving the orientation of the perpendicular magnetic recording layer and there is no mention about other characteristics. Moreover, in JP-A No. 327006/2004, a method for decreasing the medium noise is proposed by forming a first underlayer, a second underlayer, and an intermediate layer on the soft magnetic underlayer. Further, although materials having different Co compositions are described for the first underlayer, it is also a method for improving the magnetic characteristics of the perpendicular magnetic recording layer.